A GaN-HEMT (high electron mobility transistor), which has high electric breakdown field intensity of GaN and high mobility of HEMT, has promise as a high power switching element. However, so as to be driven by a driving voltage at about 10 V generated by an IC (integrated circuit), an insulating layer of a GaN-HEMT between gate and channel is configured to be thin. Therefore, the gate of a GaN-HEMT is not sufficiently insulating. Also, as a large gate capacitance makes driving of a GaN-HEMT by an IC difficult, there is an upper limit to the channel length for a GaN-HEMT. As a result, a breakdown voltage of a GaN-HEMT between source and drain is no more than about 100 V.
Thus, a GaN-HEMT provided with a field plate (hereafter, referred to as “FP”) is proposed so as to enhance the breakdown voltage (such a GaN-HEMT is referred to as a “GaN-FP-HEMT”, hereafter). (For example, refer to Wataru Saito, “Field-Plate Structure Dependence of Current Collapse Phenomena in Hight-Voltage GaN-HEMTs”, IEEE Electron device, Vol. 31, No. 7, pp. 659-661, July 2010.)